In photoelectric conversion thin-film devices, such as solar cells, it is conventionally proposed to provide a reflectance-adjustment layer for reducing reflectance, between a transparent glass substrate and a transparent conductive film in order to improve the power generation efficiency. The reflectance-adjustment layer may be a single layer or may be composed of a plurality of layers. In each layer, the refractive index of the material and the film thickness are designed based on analysis of Fresnel reflection in multiple interference films, which is known in general. The reflectance-adjustment layer has a function of eliminating differences in the in-plane reflectance spectrum, that is, hue unevenness (color unevenness), caused by in-plane film-thickness unevenness of the transparent conductive film, formed of tin oxide (SnO2), ITO, or zinc oxide (gallium doped or aluminum doped).
When photoelectric conversion thin-film devices, such as solar cells, are fabricated, it is important to set the film thickness of each layer laminated on the transparent glass substrate within an appropriate range in terms of photoelectric efficiency. PTL 1 discloses a method for measuring the film thickness of each layer every time the layer is formed, when a transparent thin film (reflectance-adjustment layer) and a transparent conductive film are laminated and formed on a transparent glass substrate.